Does it mean anything if someone prays the Rosary in his sleep?

Sounds like that could distract from the meditation. :wink:

In all seriousness, though, it seems like it could be something very good.  At least, I cannot see how this could be bad.  If it is you or somebody else who does this, my own completely non-authoritative advice is for whoever it is to continue what he is doing.  Do not fall into spiritual pride; continue to pray peacefully when awake and don't think too much about praying while asleep, otherwise you might think yourself into a mess.  Love God, contemplate His Mysteries, hope in the future, do your daily duty, and try not to get bogged down into anything else.  And if you (or whoever) continue to pray the Rosary in your sleep, praise God for such a grace !  (But don't tell anybody about it, except maybe a priest confessor.)

Then again, that is just the advice of a young layman, so take it for what it's worth.  Hopefully you find it helpful.

I don't try to pray while asleep, and I don't think I've fallen asleep while praying recently.  

I suspect that sometimes I dream about praying and then enter a state of lucidity wherein I start counting off Aves.